The invention relates to a surface wave filter and, more particularly, the invention relates to such a filter with reduced electrical cross-talk.
Surface wave filters are known e.g. through the data book "Oberflachenwillen-Filter LIOB", edition 1983/84, published by Siemens. They constitute integrated passive structural elements with band filter characteristic, whose operation is based on the interference of mechanical surface waves.
Their construction is typically of the type wherein a monocrystalline, piezoelectric substrate, in particular of lithium niobate, which has an aluminum layer vapor-deposited therein, from which piezoelectric input and output transducers are formed by photoetching. The substrate itself is fastened by glue onto a metal support. The connection of the input and output transducers with the external terminals occurs through bonding wires.
In operation, a signal fed into the input transducer is transformed into a mechanical surface wave which propagates on or in the surface of the substrate to the output transducer. At the output transducer the surface wave is retransformed into an electrical signal. The transducers generally used are interdigital structures with delay effects, which may be provided with a shielding structure. These transducers have highly frequency-dependent properties, so that based on the design of their structures filtering action for electrical signals occurs.
Although these highly desirable filter properties of surface wave filters are obtainable, they suffer from so-called cross-talk. This cross-talk results from signal transmission from the input to the output, produced by excessive electromagnetic coupling between the input and output terminals, and which also bypasses the filtering properties of the filter. In the case of high-frequency signals, with frequencies over a 100 MHz, the inductive component in particular increases greatly, so that the plastic housings presently used are not very suitable for these filters.
The undesirable rf properties of the plastic housings caused by the disadvantageous cross-talk require the utilization of expensive metal housings costly to install and to manufacture, or additional measures such as balanced transformers, shielding plates, and compensations on the circuitboard.